In general, a connector for connecting an electric wire in a motor vehicle or the like is provided with a male terminal and a female terminal, in which a metal covering layer formed of tin (Sn), an alloy of tin, or the like is disposed on an electrically conductive substrate (referred to as a substrate properly hereinafter) formed of an alloy of copper (Cu) or the like. The male terminal and the female terminal are individually housed in a housing, and are configured as a male connector and a female connector, respectively. The male terminal and the female terminal are formed of a metallic material, in which the metal covering layer formed of Sn or the alloy of Sn is formed on the electrically conductive substrate formed of Cu or the alloy of Cu through plating or the like. The metallic material is known as a high performance electric conductor with a combination of superior electrical conductivity and strength of the substrate and superior electrical connectability, corrosion resistance, and solderability of the metal covering layer (refer to Patent Documents 1 to 4 for example). The metallic material in general has an underlayer formed of nickel (Ni), cobalt (Co), iron (Fe) or the like through plating or the like with a barrier function for preventing an alloy content of zinc (Zn) of the substrate (refer to as a substrate element hereinafter) from diffusing into the metal covering layer.
In an environment of a high temperature such as an inside of an engine room of a motor vehicle or the like, an oxide film layer tends to be formed on a surface of the metal covering layer of Sn or the like on a surface of the terminal as Sn is easy to be oxidized. When the terminal is connected, the oxide film layer tends to be broken due to brittleness thereof. Accordingly, non-oxidized Sn under the metal covering layer is exposed, thereby obtaining excellent electrical connectability.
In recent years, the connector becomes a multi way type as an electronic control thereof progresses. Accordingly, it is necessary to insert or pull out a group of terminals of a male connector to or from those of a female connector with a large force. In a small space such as an inside of an engine room of a motor vehicle or the like, in particular, it is required to decrease the force for inserting and extracting due to difficulty of working.
In order to reduce the force for inserting and extracting, there is provided a method of reducing a pressure of a contact between the terminals. However, in a case where the method is adopted, a fretting phenomenon may occur between contact faces of the terminals, thereby causing a failure in electrical conduction between the terminals.
In the fretting phenomenon, the contact surfaces of the terminals slightly slide against each other due to a vibration or a variation in a temperature. Accordingly, the plating layer of Sn as a soft layer on the surface of the terminal is worn away and oxidized, thereby generating an abrasion powder with a large specific resistance. When the fretting phenomenon occurs between the terminals, connection of the terminals may be deteriorated. When a contact pressure between the terminals decreases, the phenomenon tends to occur more easily. When a thickness of the plating layer of Sn on the surface of the terminal of the connector decreases, it is possible to prevent the phenomenon somehow. However, it is still difficult to completely prevent the phenomenon.
In order to prevent the fretting phenomenon, an intermetallic compound layer of Cu—Sn such as Cu6Sn5 or the like may be formed on a base material as a hard layer (refer to Patent Document 5 and 6). However, in the method, a large amount of an element of the base material such as Cu or the like diffuses into the intermetallic compound layer of Cu—Sn, thereby making the intermetallic compound layer brittle.
Still further, there is disclosed a metallic material in which a layer of Ni is provided between the substrate and the intermetallic compound layer of Cu—Sn in order to prevent diffusion of an element from the substrate (refer to Patent Document 7). In the metallic material, no layer of Sn or Cu exists between the layer of Ni and the intermetallic compound layer of Cu—Sn. When the metallic material is produced, Ni, Cu and Sn are sequentially plated on the substrate as a layered structure, and then thermally treated. Accordingly, it is necessary to exactly design a thickness of the plating of the layered structure based on a stoichiometric proportion of Cu and Sn, and to perform the heat treatment under a strict control, thereby requiring extensive production efforts.
Furthermore, there is proposed a structure in which an intermetallic compound layer of Cu—Sn such as a Cu6Sn5 or the like is formed on a base material as a hard layer, and a particle of Sn is attached to a part of a surface of the intermetallic compound layer (refer to Patent Document 8). However, Sn is formed in a soft particle, and the fretting phenomenon still occurs between the contact faces of the terminals, thereby providing no advantage over Patent Documents 1 to 4.
[Patent Document 1] Japanese Patent Application Publication No. 2004-179055
[Patent Document 2] Japanese Patent Application Publication No. 2000-021545
[Patent Document 3] Japanese Patent Application Publication No. 2003-082499
[Patent Document 4] Japanese Patent Application Publication No. 2004-339555
[Patent Document 5] Japanese Patent Application Publication No. 2000-212720
[Patent Document 6] Japanese Patent Application Publication No. 2000-226645
[Patent Document 7] Japanese Patent Application Publication No. 2004-068026
[Patent Document 8] Japanese Patent Application Publication No. 2003-213486